Shell and tube heat exchangers are conventionally constructed with the heat exchanger tubes arranged in parallel banks or passes within the heat exchanger shell. The tubes may be straight, in which case each bank of tubes must be connected at each end with an adjoining bank, which may be beside it, above it, or below it. The heat exchanger tubes are usually of small diameter, on the order of 3/4-1 inch, and are arranged with close center-to-center spacing, generally on the order of 11/4 tube diameters. A bank may comprise hundreds of these tubes. It is not feasible to connect individual tubes in such assemblies by conventional means. Conventionally, the tubes have been fitted into a tube sheet at each end, which tube sheet forms a wall of a plenum chamber. Plenum chambers of adjoining passes have been connected by ducts.
While the conventional construction above outlined is satisfactory for many purposes, it is undesirable where the medium passing through the tubes is two-phase, such as a gas and liquid, or a slurry of solids in liquid. In the plenum chamber the constituents of such medium can separate, rendering the flow through suceeding tube banks non-homogeneous. The opportunity for phase separation may be halved by utilizing U-tubes in the heat exchanger, the tubes being arranged so that their bends connect the tube banks in adjoining passes in the same plane. It is not feasible, however, to construct heat exchangers having tubes with successive U-bends in planes normal to each other.